There are times when some players do Tri into Fester and get away with it - this is because their latency is already so high enough that their clip factor delays the Fester long enough to happen after dots have been successfully applied. When I'm lazy in pug content I can get away with this 99% of the time (on a total lack of ping spike the fester will whiff). No one should do this, but it only works for me as I'm taking advantage of an annoying situation

The openers have been crafted such to keep all these considerations in mind so that they will do what you want with consistency.